host/audio: emit initial app_audio "lost" at strict capture start (A23 P2/F1)

In strict per-app mode the default-sink loopback is suppressed, so until the
chosen app's first stream routes the viewer hears silence. Previously no event
fired for an app that never routed (`lost` only fires on an N→0 transition
after a prior route), so peerspeak couldn't warn — the share looked normal but
was silent. Emit a `lost` at capture start (lazy, on first viewer) when, and
only when, `--app` + `--strict-audio` are both set; whole-desktop and
best-effort modes keep audio flowing via the loopback and emit nothing.

Factored the emit decision into the pure, unit-tested `initial_app_audio_state`;
derive Debug/PartialEq/Eq on AppAudioState so it can be asserted on.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/host/mod.rs

@@ -535,4 +535,20 @@ mod tests {
         );
         assert_eq!(capture_summary(&opts(None, true)), "fullscreen + system-audio");
     }
+
+    #[test]
+    fn initial_app_audio_is_lost_only_in_strict_app_mode() {
+        use crate::common::output::AppAudioState;
+        use crate::host::audio::initial_app_audio_state;
+        // Strict + app: announce silence up front (loopback suppressed).
+        assert_eq!(
+            initial_app_audio_state(&opts(Some("Firefox"), true)),
+            Some(AppAudioState::Lost)
+        );
+        // Best-effort app (no strict): loopback covers the gap → no initial event.
+        assert_eq!(initial_app_audio_state(&opts(Some("Firefox"), false)), None);
+        // Whole-desktop (strict is ignored without --app): no per-app events.
+        assert_eq!(initial_app_audio_state(&opts(None, true)), None);
+        assert_eq!(initial_app_audio_state(&opts(None, false)), None);
+    }
 }